Different types of earthmoving machines are known in the prior art, with different functions related to the type of performance and power required. For instance, there are single-boom or multi-boom excavators, depending on the number of knuckle booms which connect the bucket to the machine body. A multi-boom excavator generally allows more flexibility of use compared to the single-boom type. A multi-boom excavator is generally equipped with at least one specific knuckle between two consecutive booms having a supporting boom function, wherein the relative angle of inclination between the two booms is fixed during operation but can be adjusted—generally in a standstill position—by one or more positioning cylinders connected between the two booms operated by the operator. There is also an excavation boom to which the excavating bucket is attached; the angle of the bucket can usually be changed during the excavation operations.
A multi-boom excavator with more flexibility may give rise to reliability problems. In particular, the positioning cylinder operates as a large-sized sprag, since it must withstand a strong reaction force which can cause resistance problems in the cylinder supports, in the cylinder thereof, and in its lock valve, if any. The cylinder is strongly stressed because during particularly heavy-duty excavation operations—for example when working on hard ground—the pressure of the fluid inside of it may be very high, as it is proportional to the reaction force that the cylinder needs to exert to keep the two booms in a relative fixed position. In these conditions, the cylinder supports are subjected to high stress, so much so that it could even lead to their rupture.